Because MRI is highly susceptible to motion-induced artifacts, it has been challenging to generate high-quality MRI data in the presence of patients' continual and unpredicted motion. As a result, existing abdominal MRI protocols largely rely on either respiratory gating or breath-holding to reduce motion-related artifacts. However, the respiration-gated acquisitions have low scan efficiency, particularly when the subjects have irregular breathing. Breath-hold MRI has higher scan efficiency, but may not be feasible for seriously ill patients, and the temporal acquisition window is limited by the patient's breath holding capacity.
Free-breathing MRI is a preferred protocol for abdominal imaging, particularly in challenging patients who are unable to hold their breath for an extended period of time or have irregular respiratory rates. Several approaches have been developed to reduce motion-related artifacts in free-breathing abdominal MRI data using information derived from either navigator echoes (1,2) or the over-sampled central k-space data (e.g., PROPELLER) (3,4) However, it may be difficult to use signals of low spatial resolution to effectively remove artifacts resulting from nonlinear motion. To address this concern, a series of methods have been reported recently to better model the nonlinear deformation and improve the image quality of free-breathing abdominal MRI (5,6,7).
Motion artifact issues not only affects body MRI, but can also be a major concern in neuro-MRI scans for millions of patients from highly challenging populations (e.g., children, seriously ill patients, and tremor-dominant Parkinson's patients) who currently need to rely on risky sedation or anesthesia procedures to complete lengthy neuro-MRI scans. For example: 1) a significant subset of pediatric subjects (40% of children 0-2 years of age, 75% of those 3-5 years, and 10% of those 6-17 years) need to be sedated or anesthetized in order to complete MRI procedures of 30-60 min (8); 2) Up to 37% of adult patients undergoing MRI may experience moderate to severe fear and anxiety (9), and 5 to 14% of adult patients cannot complete the MR examination in the absence of sedation or anesthesia because of claustrophobia (9-12). Existing motion artifact reduction methods, such as navigator-echo based methods and PROPELLER, may not always completely eliminate artifacts in challenging patients such as tremor-dominant Parkinson's patients.
In view of the above, there remains a need for motion-immune MRI methods where high-quality neuro and/or free-breathing abdominal MRI data can be obtained from challenging patient populations without requiring sedation or anesthesia procedures.